The proposed experiments will examine the within-trial contrast effect reported by Clement, Feltus, Kaiser, and Zentall (2000) and Clement and Zentall (2002). Clement et al. reported that when given a choice between a stimulus that followed a high effort requirement and a stimulus that followed a low effort requirement, pigeons preferred the stimuli that followed greater effort. They attributed the stimulus preference to a within-trial contrast effect in which stimuli that follow effort appear to have more value when compared to the aversive event that preceded it. Clement and Zentall found support for a contrast account by manipulating the expectation of effort. In their experiments, pigeons preferred the stimuli that were associated with a high effort initial stimulus. While their design suggests that a contrast effect can produce a stimulus preference, it did not rule out a delay reduction account or an effect of effort in Clement et al.'s design. The first set of experiments will determine whether a delay reduction account or an effect of effort could have contributed to the stimulus preference found by Clement et al. The second set of experiments will examine the effects of manipulating reinforcement on the within-trial contrast effect. [unreadable] [unreadable]